1. Field of the Invention
The invention relates to radiation sensitive silver halide emulsions useful in photography and to processes for their preparation.
2. Background
The most commonly employed photographic elements are those which contain a radiation sensitive silver halide emulsion layer coated on a support. Although other ingredients can be present, the essential components of the emulsion layer are radiation sensitive silver halide microcrystals, commonly referred to as grains, which form the discrete phase of the photographic emulsion, and a vehicle, which forms the continuous phase of the photographic emulsion.
It is important to recognize that the vehicle encompasses both the peptizer and the binder employed in the preparation of the emulsion layer. The peptizer is introduced during the precipitation of the grains to avoid their coalescence or flocculation. Peptizer concentrations of from 0.2 to 10 percent, by weight, based on the total weight of emulsion as prepared by precipitation, can be employed.
It is common practice to maintain the concentration of the peptizer in the emulsion as initially prepared below about 6 percent, based on total emulsion weight, and to adjust the emulsion vehicle concentration upwardly for optimum coating characteristics by delayed binder additions. For example, the emulsion as initially prepared commonly contains from about 5 to 50 grams of peptizer per mole of silver, more typically from about 10 to 30 grams of peptizer per mole of silver. Binder can be added prior to coating to bring the total vehicle concentration up to 1000 grams per mole of silver. The concentration of the vehicle in the emulsion layer is preferably above 50 grams per mole of silver. In a completed silver halide photographic element the vehicle preferably forms about 30 to 70 percent by weight of the emulsion layer. Thus, the major portion of the vehicle in the emulsion layer is typically not derived from the peptizer, but from the binder that is later introduced.
While a variety of hydrophilic colloids are known to be useful peptizers, preferred peptizers are gelatin--e.g., alkali-treated gelatin (cattle bone or hide gelatin) or acid-treated gelatin (pigskin gelatin)--and gelatin derivatives--e.g., acetylated gelatin or phthalated gelatin. Gelatin and gelatin derivative peptizers are hereinafter collectively referred to as "gelation-peptizers".
Materials useful as peptizers, particularly gelatin and gelatin derivatives, are also commonly employed as binders in preparing an emulsion for coating. However, many materials are useful as vehicles, including materials referred to as vehicle extenders, such as latices and other hydrophobic materials, which are inefficient peptizers. A listing of known vehicles is provided by Research Disclosure, Vol 176, Dec. 1978, Item 17643, Section IX, Vehicles and vdhicle extenders. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England.
Interest in silver halide photography has focused on twin-plane grain emulsions, particularly high aspect ratio octahedral tabular grain emulsions having two or more parallel twin planes. It has been shown that these emulsions can produce a variety of photographic advantages, including increased sharpness, improved speed-granularity relationships, increased blue and minus blue speed separations, more rapid developability, higher silver covering power when fully forehardened, reduced crossover in spectrally sensitized Duplitized.TM. (two sided) radiographic formats, and various imaging advantages in dye image transfer film units. Research Disclosure, Vol. 225, Jan. 1983, Item 22534, is considered representative of these teachings.
The formation of twin-plane silver halide crystals in photographic emulsion has been described by E. Klein et al in Fotografische Korrespondenz, 99 (7), 99-102 (1963).
Maskasky U.S. Pat. Nos. 4,713,320 and 4,713,323, are directed to tabular grain silver brom(oiod)ide (silver bromide and silver bromoiodide) and chlor(obrom)ide (silver chloride and silver chlorobromide) emulsions and processes for their preparation employing a gelatino-peptizer treated with an oxidizing agent to convert the divalent sulfur atom of the methionine to a tetravalent sulfinyl or hexavalent sulfonyl form.
The present invention offers an alternative approach for improving the characteristics of photographic silver halide emulsions prepared in the presence of methionine containing gelatino-peptizers.